In general, an organic electroluminescent device (hereinafter, referred to as organic EL device) is constructed of a light-emitting layer and a pair of counter electrodes interposing the light-emitting layer therebetween in its simplest structure. That is, the organic EL device uses the phenomenon that, when an electric field is applied between both the electrodes, electrons are injected from a cathode and holes are injected from an anode, and each electron and each hole recombine in the light-emitting layer to emit light.
In recent years, progress has been made in developing an organic EL device using an organic thin film. In order to enhance luminous efficiency particularly, the optimization of the kind of electrodes has been attempted for the purpose of improving the efficiency of injection of carriers from the electrodes. As a result, there has been developed a device in which a hole-transporting layer formed of an aromatic diamine compound and a light-emitting layer formed of an 8-hydroxyquinoline aluminum complex (hereinafter, referred to as Alq3) are provided between two electrodes as thin films, resulting in a significant improvement in luminous efficiency, compared with conventional devices in which a single crystal of anthracene molecules or the like is used. Thus, the development of the above-mentioned organic EL device has been promoted in order to accomplish its practical application to a high-performance flat panel having features such as self luminescence and rapid response.
Further, studies have been made on using phosphorescent light rather than fluorescent light as an attempt to raise the luminous efficiency of a device. Many kinds of devices including the above-mentioned device in which a hole-transporting layer formed of an aromatic diamine compound and a light-emitting layer formed of Alq3 are provided emit light by using fluorescent light emission. However, by using phosphorescent light emission, that is, by using light emission from a triplet excited state, luminous efficiency is expected to be improved by about three times to four times, compared with the case of using conventional devices in which fluorescent light (singlet) is used. In order to accomplish this purpose, studies have been made on adopting a coumarin derivative or a benzophenone derivative as a light-emitting layer, but extremely low luminance has only been provided. Further, studies have been made on using a europium complex as an attempt to use a triplet state, but highly efficient light emission has not been accomplished. In recent years, many studies centered on an organic metal complex such as an iridium complex have been made, as described in Patent Literature 1, for the purpose of attaining the high efficiency and long service life of light emission.
Citation List
Patent Literature
    [PTL 1] JP 2003-515897 T    [PTL 2] JP 2001-313178 A    [PTL 3] JP 11-162650 A    [PTL 4] JP 11-176578 A
In order to obtain high luminous efficiency, host materials that are used with the dopant materials described above play an important role. Typical examples of the host materials proposed include 4,4′-bis(9-carbazolyl)biphenyl (hereinafter, referred to as CBP) as a carbazole compound disclosed in Patent Literature 2. When CBP is used as a host material for a green phosphorescent light-emitting material typified by a tris(2-phenylpyridine)iridium complex (hereinafter, referred to as Ir(ppy)3), the injection balance between charges is disturbed because CBP has the characteristic of facilitating the delivery of holes and not facilitating the delivery of electrons. Thus, excessively delivered holes flow out into an electron-transporting layer side, with the result that the luminous efficiency from Ir(ppy)3 lowers.
In order to provide high luminous efficiency to an organic EL device as described above, it is necessary to use a host material which has high triplet excitation energy and is striking a good balance in both charge (hole and electron)-injecting/transporting property. Further desired is a compound which has electrochemical stability, has high heat resistance, and has excellent amorphous stability, and hence further improvement has been demanded.
Patent Literature 3 discloses the indolocarbazole compound shown below as a hole-transporting material.

Moreover, Patent Literature 4 discloses the indolocarbazole compound shown below as a hole-transporting material.

However, although the literature recommends to use these compounds having an indolocarbazole skeleton as hole-transporting materials, the literature only discloses examples of using each compound in a fluorescent light-emitting device, and does not disclose the use of the compounds as materials for a phosphorescent light-emitting device.